NMR spectroscopy is known as one of the most important diagnostic tools available to scientists and engineers across a wide range of fields. Therefore, this disclosure assumes familiarity with the primary aspects of NMR spectroscopy and experiments, and will only focus on the aspects most relevant to the applications described herein rather than providing an exhaustive summary. To the extent further explanation may be helpful, review of J. Keeler, Understanding NMR Spectroscopy, Wiley, 2006; B. Cowan, Nuclear Magnetic Resonance and Relaxation, Cambridge University Press, 1997; J. Kowalewski and L. Maier, Nuclear Spin Relaxation in Liquids, Taylor & Francis, 2006; or similar references can help elucidate the foundational principles of the field.
Numerous atoms with odd atomic numbers and/or odd atomic mass numbers such as Hydrogen have nonzero nuclear spin and therefore possess a nuclear magnetic moment. An atom such as Hydrogen with a spin quantum number of I=½ has two possible nuclear spin states when placed in a magnetic field as its nuclear magnetic moment orients relative to the field. In one spin state the nuclear magnetic moment orients parallel to the direction of the applied magnetic field while the other orients directly against the direction of the applied field. Under Boltzmann's law and in thermal equilibrium, there is a slight preference for that alignment that has a lower energy, meaning for a sample comprising many Hydrogen nuclei (or nuclei with similar spin properties), the overall spin population of the sample favors this state. Therefore, the overall magnetic moment of a sample is typically characterized as showing the sample has a net nuclear magnetization along the direction of the z-axis, where this axis is defined by the direction of the applied magnetic field.
When the sample is irradiated with a radiofrequency (“RF”) pulse, generating a second magnetic field, one may probe the properties of the sample by reorienting the overall nuclear magnetization vector of the sample and manipulating the relative populations of the overall spins. Often, a RF pulse will be applied to a sample such that its magnetization is moved from the z-axis into a coherent vector in the x-y (transverse) plane. Once the RF pulse is finished, however, the created nuclear magnetization in the transverse plane decays to zero in a process called transverse relaxation while the magnetization along the z-axis relaxes back to its value attained in thermal equilibrium in a process called longitudinal relaxation. The measured decay of signal in the transverse plane provides the transverse, relaxation time typically denoted as T2.
NMR experiments may be performed on a wide variety of molecules. Transverse relaxation times for molecules, however, are sensitive to molecular motions. Therefore, experiments directed to samples where some amount of the sample material has a greater degree of molecular motion compared to some other amount of the same material that is constrained or contained in some way, such as within a porous material, will observe differences in the transverse relaxation times between the constrained and unconstrained material. In these systems, quantifying the relative amounts of free, out-of-pore material and constrained in-pore material is far from straightforward. This is especially true when the material is a liquid due to factors including the distribution of liquid properties, the pore size variance and distribution, and differences in the size of liquid droplets. Therefore, typical multiexpoential fits of the experimental transverse relaxation decays are not suitable as they provide strongly model dependent answers.
Certain types of NMR experiments may also relate to measuring the kinetics of a system, such as a chemical reaction or a chemical transport phenomenon. Due to the inherent time (in the order of 10 seconds or longer) needed to load an NMR sample into the spectrometer, however, conventional techniques preclude measuring the immediate kinetics or characteristics of a system or transformation after the sample is prepared, as the chemical reaction or transformation begins to proceed before the sample is loaded in the spectrometer.
To alleviate these inefficiencies, it may be desirable to utilize a method and apparatus that accurately allows the differentiation of constrained and unconstrained materials. It may also be desirable to utilize a method and apparatus that allows initiation of a chemical reaction or other transformation only after the sample is loaded into the NMR spectrometer and ready for measurement.
The invention provides NMR methods and apparatuses that, amongst other features and advantages, address these objectives. Certain embodiments of the invention provide a method and apparatus for determining an initial amount of a substance such as a liquid contained inside a porous material and an initial amount of the substance such as a liquid present outside the porous material using a nuclear magnetic resonance spectrometer. Certain other embodiments provide a method for measuring the release kinetics of a substance such as a liquid from a porous material using a nuclear magnetic resonance spectrometer. Still other embodiments provide an apparatus and method for performing chemical reactions or other transformations in situ inside a nuclear magnetic resonance probe after a sample is loaded into a nuclear magnetic resonance spectrometer. These and other objects, features and advantages of the invention or of certain embodiments of the invention will be apparent to those skilled in the art from the following disclosure and description of exemplary embodiments.